1. Field of the Invention
This invention lies in the field of electronic equipment. More particularly it is concerned with the problems which arise in the use of electronic equipment when the power supply to the equipment fails for very short, or long times.
In most electronic equipment there are one or many latches, flip flops, adders, registers, counters, etc. When the power supply voltage drops to a very low value, or zero, the operation of the electronic circuit stops. When this happens, the actual condition of each of the elements of the electronic circuit is not known. The general manner in which the equipment is started up again, when the power does resume, is to reset all of the electronic components which are resettable, when this is done it is known exactly what the circuit condition is. Therefore, it is important when a power supply voltage outage occurs, to know first, whether the power has dropped to such a low value that the electronic devices such as gates, and comparators lose control, or whether the voltage has remained above that known minimum voltage. If the voltage remains above that value, it is not necessary to reset the equipment. If the voltage drops below that value then it is imperative to reset the equipment.
Of course, when the voltage outage is long, such as several seconds or more, there is no question but that the power-on-reset is necessary. However, when the outage may be as short at 10 or 15 milli-seconds, there is no way of telling whether the reset is necessary unless the power-on-reset circuit is designed to determine whether reset is necessary.
2. Description of the Prior Art
Many circuits have been devised for the purpose of producing a power-on-reset (POR) voltage to reset the electronic elements. In most cases such POR circuits comprise simply a series resistor and capacitor between the supply voltage V and ground. The junction between the two elements is carried to an inverting gate and the output of the gate becomes the POR voltage terminal.
In general where the power outage is of a substantial period of time, this simple circuit works fine, however for very short voltage outages there are problems which concern the rate at which voltage at the junction between the resistor and capacitor fall towards zero, in relation to the rate at which the voltage V falls towards zero. In most cases the power supply voltage has capacitance in an output filter, which causes the voltage to decrease at another rate than an instantaneous drop to zero. Of course, for very short time outages, if the voltage comes to the power supply quickly enough, the voltage then will start building up rapidly and the need for a reset voltage is absent.
Therefore, for the cases where the power outages are short-time, it becomes necessary to have an apparatus which can be designed or modified externally, to alter the time-constant of critical parts of the circuit so that it will positively identify when the power voltage drops below the selected minimum. In that case it produces a power-on-reset voltage, which remains on until the power has come back on and reached a selected magnitude. The POR voltage is then cut off, and the equipment proceeds in its proper manner starting from a known condition of reset.